The present invention relates to methods and apparatus for printing, such as ink jet or thermal transfer printing, especially non-contact printing.
Printing is one of the most popular ways of conveying information to members of the general public. Digital printing using dot matrix printers allows rapid printing of text and graphics stored on computing devices such as personal computers. These printing methods allow rapid conversion of ideas and concepts to printed product at an economic price without time consuming and specialised production of intermediate printing plates such as lithographic plates. The development of digital printing methods has made printing an economic reality for the average person even in the home environment.
Conventional methods of dot matrix printing often involve the use of a printing head, e.g. an ink jet printing head, with a plurality of marking elements, e.g. ink jet nozzles. The marking elements transfer a marking material, e.g. ink or resin, from the printing head to a printing medium, e.g. paper or plastic. The printing may be monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan, magenta, yellow, black=a process black made up of a combination of C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialised colour scheme, (e.g. CMYK plus one or more additional spot or specialised colours). To print a printing medium such as paper or plastic, the marking elements are used or xe2x80x9cfiredxe2x80x9d in a specific order while the printing medium is moved relative to the printing head. Each time a marking element is fired, marking material, e.g. ink, is transferred to the printing medium by a method depending on the printing technology used. Typically, in one form of printer, the head will be moved relative to the printing medium to produce a so-called raster line which extends in a first direction, e.g. across a page. The first direction is sometimes called the xe2x80x9cfast scanxe2x80x9d direction. A raster line comprises a series of dots delivered onto the printing medium by the marking elements of the printing head. The printing medium is moved, usually intermittently, in a second direction perpendicular to the first direction. The second direction is often called the slow scan direction.
The combination of printing raster lines and moving the printing medium relative to the printing head results in a series of parallel raster lines which are usually closely spaced. Seen from a distance, the human eye perceives a complete image and does not resolve the image into individual dots provided these dots are close enough together. Closely spaced dots of different colours are not distinguishable individually but give the impression of colours determined by the amount or intensity of the three colours cyan, magenta and yellow which have been applied.
In order to improve the veracity of printing, e.g. of a straight line, it is preferred if the distance between dots of the dot matrix is small, that is the printing has a high resolution. Although it cannot be said that high resolution always means good printing, it is true that a minimum resolution is necessary for high quality printing. A small dot spacing in the slow scan direction means a small distance between marker elements on the head, whereas regularly spaced dots at a small distance in the fast scan direction places constraints on the quality of the drives used to move the printing head relative to the printing medium in the fast scan direction.
Generally, there is a mechanism for positioning a marker element in a proper location over the printing medium before it is fired. Usually, such a drive mechanism is controlled by a microprocessor, a programmable digital device such as a PAL, a PLA, a FPGA or similar although the skilled person will appreciate that anything controlled by software can also be controlled by dedicated hardware and that software is only one implementation strategy.
One general problem of dot matrix printing is the formation of artefacts caused by the digital nature of the image representation and the use of equally spaced dots. Certain artefacts such as Moirxc3xa9 patterns may be generated due to the fact that the printing attempts to portray a continuous image by a matrix or pattern of (almost) equally spaced dots. One source of artefacts can be errors in the placing of dots caused by a variety of manufacturing defects such as the location of the marker elements in the head or systematic errors in the movement of the printing head relative to the printing medium. In particular, if one marking element is misplaced or its firing direction deviates from the intended direction, the resulting printing will show a defect which can run throughout the printing. A variation in drop velocity will also cause artefacts when the printing head is moving, as time of flight of the drop will vary with variation in the velocity. Similarly, a systematic error in the way the printing medium is moved relative to the printing medium may result in defects that may be visible. For example, slip between the drive for the printing medium and the printing medium itself will introduce errors. In fact, any geometrical limitation of the printing system can be a source of errors, e.g. the length of the printing head, the spacing between marking elements, the indexing distance of the printing medium relative to the head in the slow scan direction. Such errors may result in xe2x80x9cbandingxe2x80x9d that is the distinct impression that the printing has been applied in a series of bands. The errors involved can be very smallxe2x80x94the colour discrimination, resolution and pattern recognition of the human eye are so well developed that it takes remarkably little for errors to become visible.
To alleviate some of these errors it is known to alternate or vary the use of marker elements so as to spread errors throughout the printing so that at least some systematic errors will then be disguised. For example, one method often called xe2x80x9cshinglingxe2x80x9d is known from U.S. Pat. No. 4,967,203, which describes an ink jet printer and method. Each printing location or xe2x80x9cpixelxe2x80x9d can be printed by four dots, one each for cyan, magenta, yellow and black. Adjacent pixels on a raster line are not printed by the same nozzle in the printing head. Instead, every other pixel is printed using the same nozzle. In the known system the pixels are printed in a checkerboard pattern, that is, as the head traverses in the fast scan direction a nozzle is able to print at only every other pixel location. Thus, any nozzle which prints consistently in error does not result in a line of pixels in the slow scan direction each of which has the same error. However the result is that only 50% of the nozzles in the head can print at any one time. In fact, in practice, each nozzle prints at a location which deviates a certain amount from the correct position for this nozzle. The use of shingling can distribute these errors through the printing. It is generally accepted that shingling is an inefficient method of printing as not all the nozzles are used continuously and several passes are necessary.
As said above, this kind of printing has been called xe2x80x9cshinglingxe2x80x9d. However, printing dictionaries refer to xe2x80x9cshinglingxe2x80x9d as a method to compensate for creep in book-making. The inventors are not aware of any industrially accepted term for the printing method wherein no adjacent pixels on a raster line are printed by one and the same nozzle. Therefore, from here on and in what follows, the terms xe2x80x9cmutually interstitial printingxe2x80x9d or xe2x80x9cinterstitial mutually interspersed printingxe2x80x9d are used. It is meant by these terms that an image to be printed is split up in a set of sub-images, each sub-image comprising printed parts and spaces, and wherein at least a part of the spaces in one printed sub-image form a location for the printed parts of another sub-image, and vice versa.
Another method of printing is known as xe2x80x9cinterlacingxe2x80x9d, e.g. as described in U.S. Pat. No. 4,198,642. The purpose of this type of printing is to increase the resolution of the printing device. That is, although the spacing between nozzles on the printing head along the slow scan direction is a certain distance X, the distance between printed dots in the slow scan direction is less than this distance. The relative movement between the printing medium and the printing head is indexed by a distance given by the distance X divided by an integer.
EP-1014297 and EP-1014299 describe methods and devices for reducing banding by providing an accumulated error position which falls on a different location for each colour. To avoid as far as possible the accumulated error positions of adjacent nozzle groups coinciding in the sub-scanning direction, a selection of working nozzles is made that results in the spacing between adjacent groups of working nozzles being a number of times the nozzle pitch, whereby that number is 2 or more.
U.S. Pat. Nos. 5,940,093 and 6,068,366 describe methods for printing with a printer system, wherein a relocation error is induced in a paper transport system so as to randomise, bias or redistribute harmonic errors associated with the paper transport system of a printer system. A first subset of an addressable set of ink emitting orifices in the printhead are used to print on the print medium at a registration location. The print medium is then moved in a reverse direction a predetermined distance, and the print medium is then again advanced in the advance direction and relocated at the registration location. A second subset of the addressable set of ink emitting orifices in the printhead are then used to print on the relocated print medium at the registration location. A disadvantage of this method is that the printer system must be adapted to move the print medium in an advance and in a reverse direction.
There is a continuous requirement for improvements in printing methods and printers. In particular, there is a requirement to increase the efficiency of printing using the minimum number of passes while providing high quality.
It is an object of the present invention to provide a printing method and apparatus which provides high resolution printing at high speed with a reduced visible effect of systematic errors.
In one aspect of the present invention, a dot matrix printing method is provided for printing an image on a printing medium with reduced banding by means of a printing head. The printing head is moved, during a plurality of printing passes, with respect to the printing medium, in a fast scan direction. The method comprises the step of:
writing the image as at least two sub-images during the plurality of printing passes by mutually interstitial printing steps and/or interlacing steps. This writing step comprises moving the printing medium with a transport distance step in a slow scan direction perpendicular to the fast scan direction between the printing passes of the at least two sub-images, whereby the sum of all transport distance steps after writing one swath of each sub-image is exactly one head length. The transport distance steps are performed in at least two different step lengths.
Each sub-image of the image has a resolution which is lower than the resolution of the image. Each sub-image has a swath transition line or separation line between two subsequent printing passes for printing that sub-image. The swath transition lines for at least two sub-images, and preferably for all the sub-images, fall on a different place.
In a further aspect of the present invention, an apparatus for dot matrix printing an image on a printing medium is provided. The apparatus comprises:
a printing head, the length of the printing head in a slow scan direction being the head length,
at least one array of equally spaced marking elements on the printing head,
means for generating a first linear movement between the printing head and the printing medium in a fast scan direction perpendicular to the slow scan direction, a movement in the fast scan direction during which the print head prints being called a printing pass,
means for generating a second linear movement between the printing head and the printing medium in a slow scan direction, and
printing head driving means for driving the printing head so as to print the image as a combination of mutually interstitial printing steps and/or interlacing steps.
The means for generating the second linear movement are adapted for moving the printing medium in the slow scan direction with a transport distance step between the printing passes of the at least two sub-images, whereby the sum of all transport distance steps after writing one printing pass of each sub-image is exactly one head length. The transport distance steps are performed in at least two different step lengths.
Yet a further aspect of the present invention provides a printing head assembly, the printing head assembly comprising a plurality of neighbouring heads. Each of the neighbouring heads has at least one row comprising a plurality of marking elements. The printing head assembly is intended to be used for dot matrix printing on a printing medium an image divided in sub-images, wherein during printing the printing medium is moved relative to the printing head assembly between printing passes over a transport distance step in a slow scan direction. The sum of all transport distance steps after writing one pass of each sub-image is exactly one head length of the printing head assembly. All the heads are spread over a distance in the slow scan direction which is equal to the number of marking elements in one row, divided by the number of transport distance steps needed to reach one head length.
The present invention includes a control unit for a dot matrix printer for printing an image on a printing medium with reduced banding, by moving, relative to the printing medium, a printing head, in a fast scan direction during a plurality of printing passes, the printing head having a contiguous set of equally spaced marking elements, the marking elements available to be fired at firing moments being a set of active marking elements, the length of the active marking elements on the printing head being the head length, the control unit comprising:
means for segregating the image into at least two sub-images,
means for controlling the printing of the at least two sub-images during the plurality of printing passes by mutually interstitial printing steps and/or interlacing steps, means for controlling the movement of the printing medium relative to printing head with a transport distance step in a slow scan direction between the printing passes of the at least two sub-images, whereby the sum of all transport distance steps after writing one swath of each sub-image is exactly equal to the head length, the transport distance steps being performed in at least two different step lengths.
The present invention includes a computer program product for executing any of the methods according to the present invention when executed on a computing device associated with a printing head. The present invention also includes a machine readable data storage device storing the computer program product. The present invention also includes transmission of the computer product over a local or wide area telecommunications network.
The present invention will now be described with reference to the following drawings.